1. Technical Field
The present invention relates to a solid state laser device and a method for producing a solid state laser device, and more particularly to a solid state laser device and a method for producing a solid state laser device in which a longitudinal mode can be preferably changed to a single mode by an etalon.
2. Related Art
A solid state laser device has been known that comprises a laser diode, an Nd:YAG laser medium, a non-linear optical element, an etalon and an output mirror (for instance, see U.S. Pat. No. 5,506,860). The laser diode outputs an excited laser beam. The Nd:YAG laser medium is excited by the excited laser beam to induce and emit a fundamental wave. The non-linear optical element converts the fundamental wave to a higher harmonic wave. The etalon changes a longitudinal mode to a single mode. The output mirror forms one end of an optical resonator and transmits the outputted laser beam.
When an etalon is not inserted into an optical resonator, for instance, as shown in FIG. 3, the spectrum of an outputted laser beam has a secondary oscillating line having the wavelength of 530.8 to 531.1 nm (represented by the wavelength of a fundamental wave, and refer it to as an oscillating line of the wavelength of 1061.6 to 1062.2 nm, hereinafter) or a secondary oscillating line having the wavelength of 531.4 to 531.8 nm (this is an oscillating line of a sum frequency of the previous oscillating line and the fundamental wave has no component thereof) as well as a main oscillating line having the wavelength of 532.1 to 532.6 nm (represented by the wavelength of the fundamental wave and refer it to as an oscillating line of the wavelength of 1064.2 to 1065.2 nm, hereinafter).
When the etalon is inserted into the optical resonator, for instance, as shown by a thin full line in FIG. 4, the transmitting characteristics of the etalon exhibit a sine wave form and the maximum transmittance peak thereof corresponds to the main oscillating line having the wavelength of 1064.2 to 1065.2 nm and a longitudinal mode is changed to a single mode. That is, a spectrum “having no etalon” shown by a thin broken line in FIG. 4 is changed to a spectrum “having an etalon inserted” shown by a thick full line in FIG. 4.
However, as shown in FIG. 4, assuming that the order of interference of the maximum transmittance peak corresponding to the main oscillating line having the wavelength of 1064.2 to 1065.2 nm is m, when one of other maximum transmittance peaks corresponds to the secondary oscillating line having the wavelength of 1061.6 to 1062.2 nm (in FIG. 4, the maximum transmittance peak of the order of interference of m+3 corresponds to the secondary oscillating line), a secondary peak appears having the wavelength of 530.8 to 531.1 nm also in the outputted laser beam, and further, a secondary peak having the wavelength of 531.4 to 531.8 nm appears in the outputted laser beam.
However, since the longitudinal mode is not changed to the single mode under this state, an output is undesirably unstable due to the competition of modes.